Being in the water during a thunderstorm is hazardous because water provides a path for massive amounts of electrical energy. When lightning strikes, the resulting electrical discharge creates an immediate electrocution risk. The primary threat is not the direct strike, but the current that spreads through the water from a nearby strike point. This electrical surge can travel over a significant distance, subjecting anyone in its path to a potentially lethal voltage gradient known as step voltage.
How Lightning Interacts with Water
When lightning connects with the water surface, the electrical energy does not penetrate deeply but disperses rapidly across the surface. The current is mostly confined to the top few centimeters, spreading outward from the point of impact. Current density is highest at the strike point and decreases exponentially with distance.
This rapid dispersal creates a substantial difference in electrical potential, or voltage, between two points a short distance apart. This difference is known as step potential or step voltage, and it is the main cause of injury and death.
If a person is in the water, the electrical current can enter their body at one point and exit at another, such as across the chest. This flow of current through the body’s tissues and organs causes physiological damage. The body completes the electrical circuit between two points in the water with differing voltages.
Immediate Biological Effects on the Body
The passage of an electric current through the human body in the water, even if not a direct strike, can have immediate consequences. The most significant threat is the disruption of the body’s electrical systems, particularly the heart and lungs. Exposure to the current can instantly induce cardiac arrest, stopping the heart’s rhythm, or respiratory arrest, causing the lungs to cease function.
Electrical current flowing through the body also generates intense heat, leading to severe internal and external burns. These burns are often deep and can cause significant tissue damage that may not be immediately visible on the skin. Even relatively low currents can be lethal, especially in wet conditions where the body’s electrical resistance is lowered.
Secondary effects, such as neurological damage and loss of consciousness, can lead to drowning, which is a common fatality in water-related lightning incidents. The sheer force and heat of the discharge can also cause barotrauma, which is injury from the pressure wave, and temporary paralysis. A person in the water may be incapacitated by the shock, making them unable to swim or call for help.
Safety Zones and Distance of Danger
The perimeter of risk extends far beyond the immediate strike point. The electrical energy from a lightning strike can travel hundreds of feet or more, depending on the intensity of the bolt and the water’s conductivity. For example, a normal lightning strike is potentially lethal to a swimmer within a radius of approximately 30 meters (about 100 feet) in seawater.
The type of water significantly influences the spread of the electrical current. Seawater is an excellent conductor due to its high salt content, meaning the current can travel farther and faster, posing a risk up to 300 meters (about 984 feet) away. Fresh water, such as in lakes or pools, is a poorer conductor, which can cause the current to dissipate more slowly and potentially spread wider across the surface.
Swimming pools present a unique hazard because the current can be conducted through the pool’s plumbing, metal reinforcement, and electrical systems, even if the strike occurs nearby on the ground. The surrounding wet ground and infrastructure can transmit the charge into the water. The danger zone includes the water surface, the pool deck, and the immediate surrounding area.
Essential Prevention and Evacuation Protocols
The most important rule for lightning safety near water is: “When thunder roars, go indoors.” If you can hear thunder, the storm is close enough for lightning to strike your location. Lightning can strike from up to 10 miles away from the main thunderstorm cloud.
A standard safety guideline is the “30/30 Rule.” If the time between seeing a lightning flash and hearing the thunder is 30 seconds or less, the storm is dangerously close, and you must seek shelter immediately. Wait at least 30 minutes after the last sound of thunder or flash of lightning before returning to the water or outdoor activity.
Get out of the water and away from the shore or pool edge as quickly as possible. Safe shelter includes a substantial, fully enclosed building with plumbing and wiring, or a fully enclosed, hard-topped vehicle with the windows rolled up. Avoid seeking shelter under isolated trees, small open structures, or near metal objects like fences or railings, as these can conduct electricity.